1. Field of the Invention
The invention relates to a hardenable adhesive which contains as the active component, at least one polyol and one polyisocyanate and/or their polymers, as well as the usual additives, such as, accelerators, gluing auxiliaries, pigments and/or fillers.
2. Description of the Prior Art
Hardenable adhesives, based on the reaction products of polyetherols or polyesterols with polyisocyanates or polyisocyanate prepolymers are known and described, for example, in the "Kunstoff Handbuch" (Plastics Handbook), Vol. VII, "Polyurethanes," Vieweg-Hochtlen, Carl Hanser Publishing House, Munich, 1966, page 719 ff. However, these adhesives exhibit poor adhesion to plastic surfaces when polyetherols are used as polyol component.
The products of the reaction between polyesterols and polyisocyanates or their prepolymers do adhere better to plastic surfaces. However, their adhesive properties deteriorate under the influence of moisture or water.
U.S. Pat. No. 4,107,229 discloses a coating material which is obtained by first reacting a novolak with alkylene oxide, for example, propylene oxide, and then reacting the reaction products obtained with hydroxyalkyl methacrylates and diisocyanates in styrene as a solvent. These coating materials must be hardened with peroxide. However, these coating materials are disadvantageous in that they contain a physiologically harmful solvent and that the peroxide curing is very sensitive to impurities which may be present on the substrate to be glued and in many cases may be inhibited.
German Offenlegungsschrift No. 24 40 580 discloses heat-curable resins which are obtained by first reacting a novolak with propylene oxide and esterifying the reaction product with acrylic acid or methacrylic acid. This resin also must be dissolved in a reactive solvent, such as, for example, styrene, and cured with peroxide. If novolaks are reacted only with isocyanates, products are obtained which adhere poorly and are brittle.